Syndicate

You are here

Could we desalinate using desert evaporation

You may have heard about a technique which makes ice in an otherwise warm desert when the skies are clear at night. Dig a pit, insulate it (in olden days this was done with straw by Romans and other biblical folk) and expose it to the open, clear sky at night. During the day, cover it with reflective and insulating material. The open night sky is very cold, and energy will radiate out to it. In addition, in the low humidity, evaporation chills the water. It need not be a pit, it can be an insulated tube with high walls.

I haven't had a lot of luck finding articles about the numbers on this process, and I presume it's not particularly efficient. But I started asking, could you do it with seawater? Seawater freezes at a few degrees colder than fresh, but most importantly, the ice itself is fresh, and if extracted will have minimal salt. Ice of course floats on brine as it does on fresh water, and if the brine tank is deep enough, you won't increase the concentration of salt a great deal before you remove the ice and replace the brine (though a heat exchanger, of course.)

Most people are interested in the ice because it's cold, but it may be just as valuable because it's not brine, in areas of the world where fresh water is scarce but arid land and clear night skies are plentiful. In these areas, desalination is done with power-intensive means such as distillation and reverse osmosis, so this method need only be more efficient than these to work. The "cost" of this method, if it works, would be the insulated pits themselves, with plumbing, pumping the brine around from the sea, and mechanisms for covering the pits and extracting the ice. The infrastructure cost is high but the energy cost may be low, if we don't have to move the brine very far.

As for the ice, it could of course be sold as ice first, to be used for refrigeration, and then as irrigation water once melted. Or it could be melted in a heat exchanger with incoming brine to keep the brine pits ice-cold and working well. Then, after melting it could be used for irrigation or depending on circumstances, for washing or even drinking.

What I don't know is whether this even works OK on brine, and if it's so inefficient that you use an acre to only get a modest number of gallons, or that even the modest pumping and mechanical needs don't justify the water you get. Should it work, it could be very useful. After all, it seems we get a lot of wars caused by people fighting over water in the desert.